1. Technical Field
The present invention can be utilized in a field in which it is intended to obtain high purity cobalt sulfate that has a reduced amount of impurities, particularly calcium, magnesium and sodium, and can be used in battery materials, from an acidic hydrochloric acid solution containing cobalt.
2. Background Art
Cobalt is often included in mineral ores in co-presence with nickel, and is obtained as a co-product in nickel refining.
There are various methods for smelting this nickel or cobalt; however, in pyrometallurgical method in which mineral ores are introduced into a furnace together with a reducing agent, since cobalt is not separated from nickel and is directly smelted into ferronickel that serves as a raw material for stainless steel, cobalt loss occurs, which is not preferable.
Thus, a refining method has been conventionally used, in which method sulfur is added to mineral ores and introduced the mineral ores into a furnace to perform sulfurizing roasting, sulfide of nickel and cobalt thus obtained is leached in sulfuric acid, and the leachate thus obtained is subjected to a method such as solvent extraction, and thereby collecting cobalt as metal or a salt. However, this method of combining sulfurizing roasting and acid leaching has a problem of increases in effort and cost accompanied by an increase in the quantity of material required for the treatment, in connection with the downgrading of raw material ores that comes along with a drain on resources of the recent years.
Particularly, when leaching is performed using sulfuric acid, there has been a problem that the scale of facilities is expanded because time is required for leaching, and unless sulfuric acid generated after metal is collected is discharged out of the system, the sulfuric acid balance of the process cannot be maintained, while it is necessary to supply sulfur needed for the sulfurizing roasting described above.
Thus, recently, a method of leaching low-grade nickel ores with a sulfuric acid solution at a high temperature and a high pressure, separating impurities, subsequently adding a sulfurizing agent thereto to obtain a sulfide, leaching this sulfide with a chloride, and obtaining cobalt metal therefrom, has been put to practical use.
When this method is used, there is an advantage that since leaching of sulfide with a chloride proceeds rapidly, the facilities are relatively compact and satisfactory. In addition to that, the sulfur obtainable as leaching residue after the leaching of a sulfide with a chloride can be recycled as a raw material of the sulfurizing agent, and since electro winning by means of a chloride bath has higher conductivity than a sulfuric acid bath, the electric power for electrolysis can be saved. Also, the chloride ions obtained after collecting cobalt metal can be recycled to the leaching process. Thus, it is known be advantageous in view of being efficient, cost-saving, and effort-saving.
However, when the method is used, metal can be collected efficiently; however, it is not easy to obtain salts of cobalt, particularly cobalt sulfate.
Particularly, in recent years, cobalt sulfate has been used in large quantities as a material for secondary batteries, but there are requirement specifications of cobalt sulfate for exclusive use in batteries for the purpose of securing battery characteristics or securing safety. Among others, it is necessary to maintain the amount of chloride ions in the cobalt sulfate crystals generally at a level of 0.1% or less.
In regard to such harsh specifications, for example, a method of purifying cobalt metal once, and dissolving this purification product in sulfuric acid to obtain cobalt sulfate may be conceived, but when the time and cost required for production are considered, the method is not economically efficient. As such, it has been not so easy to obtain cobalt sulfate from a chloride.
Thus, a method of subjecting a chloride solution containing cobalt to solvent extraction to extract cobalt ions, subjecting these cobalt ions to be stripped with a sulfuric acid solution, and thereby obtaining a cobalt sulfate solution, can be conceived.
As an extractant capable of separating the cobalt, extractants such as phosphonic acid and phosphinic acid are known. Among specific examples of the phosphonic acid and phosphinic acid, 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester and di(2,4,4-trimethylpentyl)phosphinic acid have been generally used since these compounds have satisfactory cobalt extractability.
When an extractant such as phosphonic acid or phosphinic acid is used, the extraction behavior is dependent on the pH of the solution, and there is a tendency that as the pH increases, the extraction rate increases. Furthermore, it has been contemplated that cobalt and other impurity elements can be extracted into an organic solvent by utilizing the characteristic that the pH dependency appropriate for extraction varies with the element.
Japanese Examined Patent Publication No. 63-50411 describes a method of collecting nickel and cobalt from nickel and cobalt alloy scrap, characterized in that an alloy scrap which contains nickel and cobalt but does not contain other elements in the form of compound, is electrolytically dissolved using an aqueous sulfuric acid solution as an electrolyte liquid while maintaining the negative electrode current density at 2 A/dm2 or less, the aqueous solution containing nickel sulfate and cobalt sulfate thus obtained is subjected to a solution purification treatment, nickel ions and cobalt ions are extracted into an organic substance from the purified aqueous solution, nickel and cobalt ions are stripped from the extract liquid using hydrochloric acid or sulfuric acid, and thereby a mixed aqueous solution of nickel chloride and cobalt chloride or a mixed aqueous solution of nickel sulfate and cobalt sulfate is obtained.
When this method is used, an aqueous solution of cobalt of a chloride bath or a sulfuric acid bath can be obtained from cobalt of a sulfuric acid bath.
However, in the chloride solution containing cobalt, impurities such as calcium, magnesium and sodium are also co-present. The extraction behavior of these impurities in the extractants described above has a property similar to the extraction behavior of cobalt, so that it has been difficult to remove impurities such as calcium, magnesium and sodium from a solution containing cobalt.
In regard to the method of separating impurities, Japanese Unexamined Patent Application Publication No. 2010-43313 discloses a method of promoting an enhancement in the copper extraction ability when copper is solvent extracted from a chloride bath.
This method is a method of collecting copper by solvent extraction from an acidic aqueous solution containing copper chloride and chlorides of alkali metals and/or alkaline earth metals, using a cation exchange type extractant, and solvent extraction is carried out in the presence of sulfate ions. A sulfuric acid compound selected from the group consisting of sodium sulfate, magnesium sulfate, calcium sulfate, potassium sulfate and ammonium sulfate is added to the acidic aqueous solution, and the content of sulfate ions is adjusted to the range of 10 g/L to 100 g/L.
Furthermore, the chlorine ion concentration and the bromine ion concentration in the acidic aqueous solution can be adjusted to predetermined ranges, and an acidic chelate extractant can be used as a cation exchange type extractant. In addition, it is disclosed that the copper extraction ability is increased, and the amount of solution handled in the copper leaching process carried out in the previous step can be reduced, so that the facility cost, operation cost and the like can be reduced.
However, a method of easily separating impurities such as calcium, magnesium and sodium in an industrially effective manner from a solution mainly containing cobalt, has not yet been found.
An object of the present invention is to produce high purity cobalt sulfate by effectively eliminating impurities such as calcium, magnesium and sodium in a process for obtaining a cobalt sulfate solution having a high cobalt concentration by solvent extraction using an acidic organic extractant.